1. Field of the Invention
The present invention relates to a sample resistance measurement device for measuring an electrical connection state of a circuit component such as an IC soldered and mounted on a circuit substrate for performing an environmental test such as a vibration test, a shock test, or the like, and particularly relates to a sample resistance measurement device for measuring minute resistance variation of a soldered point of the circuit component caused in an environmental test by utilizing Wheatstone bridge.
2. Description of the Related Arts
Conventionally, in an information processing device such as a computer, circuit components such as a CPU and a memory are mounted on a circuit board by soldering. Moreover, recently, package structures of integrated circuits devised for realizing high-density mounting are also used. As leadless package structures suitable for such high-density mounting of integrated circuits, for example, a ball grid array (Ball Grid Array) and a QFN package (Quad Flat Non-Leaded Package) are known. The ball grid array commonly known as BGA is a package in which leadless terminals composed of solder balls are disposed in a grid pattern at a constant interval on the rear surface of the package which is in contact with a printed board, and the leadless terminals are soldered to the corresponding pattern of the printed board. The leadless terminals are on the rear side of the package and in a narrow space sandwiched by the package and the printed board; therefore, a solder paste for joint is applied in advance on the pattern of the printed board in a printing process, and soldering is performed by heating the entire package and melting the solder for joint applied on the part of the terminals. The QFN package is a package having a structure in which leads are eliminated, and merely electrode pads for soldering are formed, and the electrode pads are positioned on the rear surface of the package. On the other hand, various environmental tests of, for example, a vibration test, a shock test, and thermal shock are performed by environmental test equipment for the circuit boards having circuit components mounted on printed boards by soldering, thereby ensuring reliability and durability thereof when they are shipped as products. In such environmental tests, thermal stress and mechanical stress is repeatedly applied to various connecting parts of the circuit parts mounted on the circuit boards so as to generate distortion therein, and they are mechanically separated and cut when the stress reaches limit, thereby causing temporary interruption in which electrical connection is temporarily disconnected. In the environmental tests, conventionally, at the end of the tests, whether exfoliation, cracks, and the like are generated or not is checked by visually inspecting the soldered parts of the circuit components. However, in the BGAs and QFNs used in recent high-density mounting, the soldered parts cannot be seen from outside, and visual inspection cannot be performed. Therefore, in the environmental tests of the circuit boards in which the soldered parts cannot be seen from outside, resistance measurement has to be performed as a method for measuring the state of electrical connection of the circuit components. In the resistance measurement, a method using a data logger and a method using an oscilloscope of high-speed sampling are known. On the other hand, as a method for measuring distortion, a method in which a Wheatstone bridge and a high-speed amplifier (abbreviated as “dynamic amplifier”) for capturing dynamic phenomena are combined is known. This method captures variation in distortion amount as variation in resistance value at high speed.
FIG. 1A is a two-wire type data logger; wherein a sample 104 is connected to cables lead from a resistance meter 100, a constant voltage is fed from the resistance meter 100 to the sample 104, and the flowing current is detected so as to measure the resistance of the sample 104. In the case of the two-wire type data logger, measured resistance includes wiring resistance of the cable 102, and the resistance of the sample 104 cannot be accurately measured.
FIG. 1B is a four-wire type data logger; wherein cables 108 of the four-wire type are lead from a resistance meter 106 and connected to the sample 104, a constant voltage is fed from the resistance meter 106 to the sample 104, and the flowing current is detected so as to measure the resistance of the sample 104. In this case, by virtue of the four-wire type, the measured resistance does not include the wiring resistance of the cables 108, and the resistance of the sample 104 can be accurately measured.
FIG. 2 is an oscilloscope 110 of high-speed sampling; wherein cables 112 connecting the sample 104 are connected to a constant-voltage power supply 114 and connected to a current meter 116 for monitoring in the side of the oscilloscope 110.
FIG. 3A is a dynamic distortion measurement device using a Wheatstone bridge which is comprised of a Wheatstone bridge 118, a dynamic distortion amplifier 120, a distortion gauge 122, a constant-voltage power supply 126, and a dynamic distortion processing device 128. The distortion gauge 122 is connected as one resistance of a bridge side of the Wheatstone bridge 118, all the bridge resistances including the distortion gauge 122 are set to be a same resistance value R in a state in which a bridge input voltage Ei is applied from the constant-voltage power supply 126, and a bridge output voltage Eo with respect to the dynamic distortion amplifier 120 is 0 volt. The distortion gauge 122 is fixed like FIG. 3B on a sample 124 for which a vibration test is to be performed; wherein the resistance value is increased when the sample 124 is upwardly curved and the distortion gauge 122 is stretched, and, inversely, the resistance value is decreased when the sample 124 is downwardly curved and the distortion gauge 122 shrinks. In accordance with such variation of the resistance value of the distortion gauge 122, the bridge output voltage Eo is varied. The bridge output voltage Eo is input to the dynamic distortion amplifier 120, and the amplifier outputs a carrier wave of the current which is changed in accordance with the bridge output voltage Eo and performs high-speed sampling of the voltage variation due to output of the carrier wave by an A/D converter. Therefore, although the signal lines from the Wheatstone bridge 118 to the sample 124 are two lines, the amount of the changed resistance due to vibration of the distortion gauge 122 provided on the sample 124 can be extracted.
However, the stress generated in the connecting part of the sample during an environmental test in which a vibration test, a shock test, or the like is performed is generated as a transient phenomenon, and a response speed for capturing variation of a frequency range of about 10 Hz to 2 KHz is required. Therefore, the measurement sampling speed is required up to 50 Hz to 10 KHz which are about five times the response speed. Both the two-wire type and the four-wire type data loggers have measurement response time of 0.1 second at minimum and a response speed of merely a 10 Hz level; therefore, they are completely deficient in capability. Moreover, in the case of the two-wire type data logger, measured resistance includes the wiring resistance of the cables connecting the resistance meter to the sample, and resistance variation of the sample cannot be measured. Furthermore, in the four-wire type data logger, although measured resistance of the sample does not include wiring resistance, the number of wires is increased. In the oscilloscope of high-speed sampling, constant-current power supplies as many as the number of channels are required, and the scale of the measurement system is increased. Therefore, the number of channels measured at the same time is about four channels at most, and it is not suitable for multi-channel measurement in an environmental test in which 10 or more channels are required. Moreover, when measured data results in distortion/vibration waveforms, the amount of data is increased, thereby causing a problem in long-time recording. On the other hand, the dynamic distortion measurement device using the Wheatstone bridge amplifies minute resistance variation of the distortion gauge at high magnification and, furthermore, is designed for multi-channels. A normal sampling cycle is 10 to 20 KHz, it is capable of long-time recording, and the ability of response is also satisfactory as a recording system. However, the dynamic distortion measurement device using the Wheatstone bridge is a device which extracts the varied amount of the resistance due to vibration from the distortion gauge fixed to a measurement object and converts the resistance varied amount into a distortion physical amount, and involves a problem that the resistance value variation of the sample for which an environmental test or the like is being performed cannot be directly monitored with high sensitivity.